Given a metric space ℳ = (X,δ), a weighted graph G over X is a metric tspanner of ℳ if for every u,v ∈ X, δ(u,v) ≤ δ_G(u,v) ≤ t⋅ δ(u,v), where δ_G is the shortest path metric in G. In this paper, we construct spanners for finite sets in metric spaces in the online setting. Here, we are given a sequence of points (s₁, …, s_n), where the points are presented one at a time (i.e., after i steps, we have seen S_i = {s₁, … , s_i}). The algorithm is allowed to add edges to the spanner when a new point arrives, however, it is not allowed to remove any edge from the spanner. The goal is to maintain a tspanner G_i for S_i for all i, while minimizing the number of edges, and their total weight.
Under the L₂norm in ℝ^d for arbitrary constant d ∈ ℕ, we present an online (1+ε)spanner algorithm with competitive ratio O_d(ε^{d} log n), improving the previous bound of O_d(ε^{(d+1)}log n). Moreover, the spanner maintained by the algorithm has O_d(ε^{1d}log ε^{1})⋅ n edges, almost matching the (offline) optimal bound of O_d(ε^{1d})⋅ n. In the plane, a tighter analysis of the same algorithm provides an almost quadratic improvement of the competitive ratio to O(ε^{3/2}logε^{1}log n), by comparing the online spanner with an instanceoptimal spanner directly, bypassing the comparison to an MST (i.e., lightness). As a counterpart, we design a sequence of points that yields a Ω_d(ε^{d}) lower bound for the competitive ratio for online (1+ε)spanner algorithms in ℝ^d under the L₁norm.
Then we turn our attention to online spanners in general metrics. Note that, it is not possible to obtain a spanner with stretch less than 3 with a subquadratic number of edges, even in the offline setting, for general metrics. We analyze an online version of the celebrated greedy spanner algorithm, dubbed ordered greedy. With stretch factor t = (2k1)(1+ε) for k ≥ 2 and ε ∈ (0,1), we show that it maintains a spanner with O(ε^{1}logε^{1})⋅ n^{1+1/k} edges and O(ε^{1}n^{1/k}log² n) lightness for a sequence of n points in a metric space. We show that these bounds cannot be significantly improved, by introducing an instance that achieves an Ω(1/k⋅ n^{1/k}) competitive ratio on both sparsity and lightness. Furthermore, we establish the tradeoff among stretch, number of edges and lightness for points in ultrametrics, showing that one can maintain a (2+ε)spanner for ultrametrics with O(ε^{1}logε^{1})⋅ n edges and O(ε^{2}) lightness.
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A Family of Metrics from the Truncated Smoothing of Reeb Graphs
In this paper, we introduce an extension of smoothing on Reeb graphs, which we call truncated smoothing; this in turn allows us to define a new family of metrics which generalize the interleaving distance for Reeb graphs. Intuitively, we "chop off" parts near local minima and maxima during the course of smoothing, where the amount cut is controlled by a parameter τ. After formalizing truncation as a functor, we show that when applied after the smoothing functor, this prevents extensive expansion of the range of the function, and yields particularly nice properties (such as maintaining connectivity) when combined with smoothing for 0 ≤ τ ≤ 2ε, where ε is the smoothing parameter. Then, for the restriction of τ ∈ [0,ε], we have additional structure which we can take advantage of to construct a categorical flow for any choice of slope m ∈ [0,1]. Using the infrastructure built for a category with a flow, this then gives an interleaving distance for every m ∈ [0,1], which is a generalization of the original interleaving distance, which is the case m = 0. While the resulting metrics are not stable, we show that any pair of these for m, m' ∈ [0,1) are strongly equivalent metrics, which in turn gives stability of each metric up to a multiplicative constant. We conclude by discussing implications of this metric within the broader family of metrics for Reeb graphs.
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 NSFPAR ID:
 10295931
 Editor(s):
 Buchin, Kevin; Colin de Verdi\` ^{}
 Date Published:
 Journal Name:
 Forensic toxicology
 Volume:
 189
 Page Range / eLocation ID:
 22:122:17
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Given a metric space ℳ = (X,δ), a weighted graph G over X is a metric tspanner of ℳ if for every u,v ∈ X, δ(u,v) ≤ δ_G(u,v) ≤ t⋅ δ(u,v), where δ_G is the shortest path metric in G. In this paper, we construct spanners for finite sets in metric spaces in the online setting. Here, we are given a sequence of points (s₁, …, s_n), where the points are presented one at a time (i.e., after i steps, we have seen S_i = {s₁, … , s_i}). The algorithm is allowed to add edges to the spanner when a new point arrives, however, it is not allowed to remove any edge from the spanner. The goal is to maintain a tspanner G_i for S_i for all i, while minimizing the number of edges, and their total weight. Under the L₂norm in ℝ^d for arbitrary constant d ∈ ℕ, we present an online (1+ε)spanner algorithm with competitive ratio O_d(ε^{d} log n), improving the previous bound of O_d(ε^{(d+1)}log n). Moreover, the spanner maintained by the algorithm has O_d(ε^{1d}log ε^{1})⋅ n edges, almost matching the (offline) optimal bound of O_d(ε^{1d})⋅ n. In the plane, a tighter analysis of the same algorithm provides an almost quadratic improvement of the competitive ratio to O(ε^{3/2}logε^{1}log n), by comparing the online spanner with an instanceoptimal spanner directly, bypassing the comparison to an MST (i.e., lightness). As a counterpart, we design a sequence of points that yields a Ω_d(ε^{d}) lower bound for the competitive ratio for online (1+ε)spanner algorithms in ℝ^d under the L₁norm. Then we turn our attention to online spanners in general metrics. Note that, it is not possible to obtain a spanner with stretch less than 3 with a subquadratic number of edges, even in the offline setting, for general metrics. We analyze an online version of the celebrated greedy spanner algorithm, dubbed ordered greedy. With stretch factor t = (2k1)(1+ε) for k ≥ 2 and ε ∈ (0,1), we show that it maintains a spanner with O(ε^{1}logε^{1})⋅ n^{1+1/k} edges and O(ε^{1}n^{1/k}log² n) lightness for a sequence of n points in a metric space. We show that these bounds cannot be significantly improved, by introducing an instance that achieves an Ω(1/k⋅ n^{1/k}) competitive ratio on both sparsity and lightness. Furthermore, we establish the tradeoff among stretch, number of edges and lightness for points in ultrametrics, showing that one can maintain a (2+ε)spanner for ultrametrics with O(ε^{1}logε^{1})⋅ n edges and O(ε^{2}) lightness.more » « less

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